
You are absolutely justified but she will likely defend him. It’s nothing personal, it’s just likely not the first time he has hit her and she had to find ways to keep going which typically excuses his actions towards her. You need to be patient with her and continue to be firm with him.
Don’t argue with her about it or get upset, it will only make her feel less comfortable with coming to you about things like this in the future. Talk to her softly and just ask questions. Dont make demands or give any ultimatums and don’t try to force her to leave (unless you believe he’s going to try to kill her soon) because it will ultimately likely make her cut contact with you and the family (you can still encourage her to leave but not forcefully)